Process for producing oxide coatings on aluminum and aluminum alloys



United States Patent PROCESS FOR PRODUCING OXIDE COATINGS 0N ALUMHJUM AND ALUMINUM ALLOYS John B. Franklin, Norwalk, Califi, assignor to Sanford Process Co., Inc., Los Angeles, Calif., a corporation of California No Drawing. Application October 8, 1956 Serial No. 614,388

17 Claims. (Cl. 204-42) This application is a continuation-in-part of my copending application Serial No. 438,349, filed June 21 1954.

This invention relates to the production of hard, smooth, wear and corrosion resistant aluminum oxide films on aluminum and aluminum alloys by electrolytic oxidation of the aluminum and the aluminum alloys.

In my above co-pending application, I have disclosed and claimed a process capable of making thicker, denser and harder oxide coatings on aluminum and aluminum alloys than prior art processes known to me, and also produces thick, dense, hard oxide coatings on aluminum alloys which prior art processes are incapable of coating and is substantially insensitive to the presence of copper and iron ions in the bath. Thus, according to the process of the above application oxide coating thicknesses up to .010" or more can be obtained. Improved bonding of the oxide coating to the metal is also obtained, and the coating is deposited more rapidly than heretofore in prior art processes.

While the process of my co-pending application possesses several advantages over prior art processes, a substantial portion of that period during which the part is anodically oxidized according to the process of my copending application is consumed in forming the initial oxide coating. Thus, for example, in producing an initial coating of .0005" thickness on say 245, 618 or 755 aluminum alloy, a period of about 25 to 40 minutes may be required according to the process of my co-pending application to raise the voltage from a little over 20 volts through the critical voltage (voltage at which measurable oxide coating is formed), and up to the voltage value required for deposition of a .0005" coat. In addition, care must be taken throughout this initial period to maintain a relatively low amperage during the stepwise increase of the voltage according to the technique of my co-pending application, in order to prevent burning of the part or destruction of the oxide coating being formed. 7 I have now found that by commencing the electrolytic oxidation at a bath temperature of about 40 to about 65 F., and maintaining such temperature until a coating of at least about .0002" and not more than about .002 is produced, and then continuing the anodizing process in an electrolyte at a lower temperature than about 40 F., but not less than about 0 F., and preferably employing at this lower temperature the voltage increment technique of my co-pending application, the overall time required to produce a coating of a given thickness can be materially reduced, e.g., by about one-third. When it is desired to obtain smoother coats, I limit the coating thickness formed at the higher temperature of about 40 to about 65 F., to from about .0005" to about .0015", and for most practical operation to between about .0005" and .001", most desirably about .0005" thick. Thus, by practicing the instant process, the time required, e.g., for

obtaining a .005" oxide coating, can be out say from about 70 minutes to about 45 minutes. According to the instant improvement, an initial coating of .0005" can be deposited in from 4 to 5 minutes compared to the about 25 minutes or more required according to the procedure of my co-pending application.

Briefly then, according to a preferred embodiment,

my process involves anodizing the aluminum or alui minum alloy part in an electrolyte maintained at about' 40 to about F. until a .0005" oxide coat is formed, and then transferring the part to another electrolyte bath maintained at a lower temperature preferably between about 0 F. and about 35 F. to complete oxidation to the desired thickness of coating. During the anodizing pe riod in the initial electrolyte at the higher about 40-65 F. temperatures, voltage is raised, while current density is at first permitted to rise and is then caused to drop to,- ward the latter portion of this period. During the, anodizing period in the second electrolyte at the lower temperature, voltage is increased while current density,

is maintained within a certain range, the upper limit of which is generally not greater than the current density value at the end of the anodizing period in the initial elec-' trolyte, all as described more fully below.

If the final coating desired is greater than .002 thicke. ness, the process may be continued in the initialelecw trolyte at the higher temperature of about 40.65 F. until a .002 thick coating is formed, and the additional coating should be formed in the electrolyte at reduced. temperature preferably not in excess of 35 F., e.g., about,

Further, it is known that aluminum anodizing generally causes an increase in surface roughness of the oxide coating over its initial degree of roughness prior to anodizing.

The technique of the instant process has the additionaladvantage that it results in a smoother and harder coat-. ing than that obtained in prior art processes, including that of my co-pending application. Thus, in the instant process the anodized coating may be almost as smooth as the metal surface prior to anodizing, or only slightly rougher, whereas according to prior techniques, the final anodized surface is ordinarily considerably rougher than the original surface.

In preparing the electrolyte I may employ any of the acids usually employed in making up the electrolyte for electrolytic oxidation of aluminum such as sulphuric acid, chromic acid, or oxalic acid or mixtures thereof. The latter acids are considered equivalents for the anodic ox idation of aluminum and its alloys and are termed elec tro-anodizing acids herein. Other acids, in addition to sulphuric, oxalic and chromic acids, have been suggested by the prior art for the anodic oxidation of aluminum, and those skilled in the art will understand the nature and type of such acids contemplated herein.

I may use up to say about H in the electro lyte, but I prefer to use dilute H 80 in an amount corresponding to a range of from about 1 part to 20 parts by volume of concentrated sulphuric acid dissolved in 100 parts by volume of water. For example, I may employ an amount of H 80 in the electrolyte corresponding to from about to 10% by volume of 66. Baum sulphuric acid per 100 parts by volume of water. I also preferably employ in the bath from 1 to 6 parts, preferably about 3 to about 6 parts, by volume of an aqueous extract of peat per 100 parts by volume of water in the anodizing acid solution.

In order to prevent freezing of the electrolyte at the, low temperatures employed I may add an alcohol soluble n t e rqlyts. uch methyl alcohol o a y m ter l.

2,905,600 PatentedSept. 22, 19,59

sa anna which will lower the freezing point of the solution. How-- ever, ordinarily such an anti-freeze is not necessary.

It is understood that the above proportions of ingredients in the electrolyte may be varied if desired.

The extract additive can be obtained by extracting peat obtained from various localities, with water. The extraction is particularly made more rapid and yield is increased by extraction at elevated temperature, preferably the atmospheric boiling point of the mixture or more elevated temperature, and most desirably by extraction at elevated temperature under pressure for a period sufiicient to produce an aqueous acid solution. The peat used in practice of my invention may be derived from various locations in the United States, for example, from Georgia, Florida, California or Michigan.

In practice, the peat is first mixed with water in a proportion of say one part by weight of the ground, flaked or fibrous .peat with, for example, six parts of water. These proportions may vary, however. This aqueous mixture is then fed to an autoclave when high extraction temperatures are desired for practical reasons and wherein the mixture is cooked at autogenous pressure and at temperature above its normal boiling point (i.e., the boiling point of the mixture at atmospheric pressure). In this respect I have found that satisfactory results according to the invention are obtained by cooking the peat at a temperature of from about the boiling point of the mixture at atmospheric pressure, to about 290 F. or higher, e.g., between about 250 F. and about 350 F., and at pressures up to about 140 p.s.i. for from about 6 to 100 hours or more, but preferably less than 100 hours.

While the above described high pressure and high temperature method for producing the extract is preferred, I can also obtain such extract by refluxing the aqueous mixture of peat at atmospheric pressure over an extended period say over 100 hours. After extraction, the undissolved residue may be separated from the extract.

Preferably, I employ an aqueous extract of Georgia peat made according to the example described below.

Four parts by volume of Georgia peat ground and mixed is added to 30 parts of water and the resulting mixture is cooked at superatmospheric pressure between and 20 pounds gage pressure for about 72 hours. The resulting material is cooled and allowed to settle. The liquid is decanted from the insoluble residue and forms the peat extract additive which is incorporated in the electrolyte according to the invention. This extract is in the form of an aqueous solution of organic acids of a complex nature, and is characterized by the following properties. The aqueous extract has a pH of from about 4 to about 6 and may contain as little as 2% of dissolved or dispersed solids, depending on the amount of dilution of the material. The extract upon evaporation to dryness leaves a dark brown, glossy residue which is amorphous and has a total nitrogen content of about 4.5 to 5%, for example, 4.8% as determined by the Kieldahl method. The extract solids are essentially water soluble and form a clear dark brown solution or dispersion. The aqueous extract is preferably kept refrigerated or a small amount of sulfuric acid can be added or a fungicidal material such as Dowicide A, believed to be essentially sodium o-phenyl phenate and marketed by the Dow Chemical Company, may be added to prevent mold growth.

The part to be coated is connected to the anode of an electrolytic cell and immersed in the electrolyte bath which is maintained during the first stage of the reaction at the above noted temperature of between about 40 and about 65 F., preferably 45 to 55 F. The second stage of the reaction is carried out at electrolyte temperatures ranging from about 0 F.- to about 35 F. ordinarily, and preferably 10 to 30 F. Direct current for anodizing is applied in both the first and second stages. If desired, however, an alternating current component m y added to the direct current. As the coating becomes thicker, its electrical resistance requires higher voltages for penetration. I have found that the addition of peat extract to the electrolyte is necessary in conjunction with my technique for obtaining the improved results noted herein. The presence of the peat extract aids in preventing burning of the part at high voltages and amperages, and also permits use of low electrolyte temperatures, particularly in the second stage electrolyte of the invention. The rapid increase of voltage at the end of the coating period produces the proper thickness quickly with little softening or solution of the aluminum oxide coating; hence, harder and thicker and also smoother coatings are obtained by my process than are obtainable by the prior art.

In operating my process I raise the voltage say to about 20 volts and thereafter increase the voltage until the initial coating is formed. In this period of operation wherein the bath temperature is maintained between about 40 and about 65 F., I preferably raise the voltage gradually from the initial voltage of 20 volts to 26 volts in about 1 minute, measuring the voltage across the cell electrodes and depending somewhat on the cell resistance. Thereafter the voltage is raised in approximately 1 volt increments, each for a period of between about 15 and 45 seconds until the initial coat is formed. For most wrought aluminum alloys such as 248, 618, and 758, the voltage at which a .0005" thick oxide coat is formed is between about 31 and 34 volts and will be higher if the coat is greater in thickness or lower if the initial coat is thinner than .0005". Hence, it is seen that a coat of .0005 can be formed in from about 4 to about 7 minutes from the commencement of the electrolytic oxidation.

In forming the initial coat, voltage is gradually increased up to about 20 to 26 volts depending on the alloy, and amperage increases gradually with voltage, to about 50 to 60 amperes per square foot at about 26 volts, the actual amperage increase depending on the resistances in the system. Thereafter, voltage is raised in approximately- 1 volt increments as described above, while current density follows an overall decreasing trend until a desired initial coating thickness, e.g., .0005", is built up. During each voltage step, as the voltage is raised the current is increased, and while during said voltage step the voltage is maintained substantially constant the current decreases during each voltage increment, as the voltage is raised above 26 volts in the aforesaid cell, up to the voltage at which the initial, e.g., .0005? coat is formed. Thus, at the start of each such voltage increment, current density increases and thereafter drops. However, the voltage increment is adjusted so that amperage at the start of each of the successive voltage steps is less than the amperage at the start of the previous voltage step, until at the voltage at which, for example, a .0005" coat is formed, amperage falls so that the current density is down to about 20-25 amps. per square foot in the above system. The reason for adjusting the maximum amperage at each voltage step so that amperage decreases in the latter stage of this operation, e.g., from 60 amps. per square foot at about 26 volts to say about 20 amps. per square foot at the voltage at which a .0005" coat is formed, e.g., 31 to 34 volts, is that the oxide coat which commences to form at about 26 volts continues to increase in thickness as voltage increases. Thus, if amperage where permitted to increase materially or to remain constant at 50-60 amps. during this build up of oxide coating above, for example, 26 volts, the coating would tend to dissolve and the part would begin to burn. By decreasing the current density during this coat forming period above the initial voltage and up to formation of the initial coat, burning is prevented and the coating builds up in a satisfactory manner with improved hardness and smoothness.

When the initial coat of .0002-to .002" thickness is formed, the part being treated is preferably removed to 2,905,:eoo

another tank of electrolyte similar'to that employed-in the initial stage described above, but maintained at a. lower temperature down to about F., say at about 15 F. However, if desired, after formation of the initial oxide coating, say .0005, in the first stage at the higher temperature of about 40 to about 65 F., the temperature of the bath may be lowered to 35 F. or below, and anodic oxidation continued in the same bath at the lower temperature. From this point on, the coating is increased in thickness to the desired depth according to the procedure in my copending application as described more fully below.

However, if it is desired to form a final coating greater than an initial coating of about .0015" to .002" in thickness, the electrolytic oxidation may be continued in the first stage electrolyte at about 40 to about 65 F. until the initial coating is formed before carrying out the second stage operation at the lower temperature usually below 35 F. Thus, for example, after a .0005" coat is formed, voltage is increased in one volt increments, each for a duration of about 15 to 45 seconds until the desired initial thickness, e.g., up to .0015" to .002 is reached. During this period amperage decreases moderately from the amperage at the .0005" coat thickness, and at a thickness say of .0015", the amperage will decrease until during the last voltage increment amperage may range from about to 20 amperes per square foot. At the start of each voltage increment the amperage will rise but will then start to fall, the overall trend being to gradually decrease the amperage to the above noted values. With alloys such as 248 and 758, a thickness of .0015" is attained at a voltage of between about 40 and 50 volts.

Hence it will be seen that to obtain an oxide coating of, for example, .0015" thickness, this can be produced according to the instant process in from about to 20 minutes, whereas employing the technique of my copending application, a eriod of about 40 minutes may be required for this purpose. However, if I desire to form a coating of say .001" to .0015" total thickness,"I preferably do not keep the part in the electrolyte at about 40 to about 65 F. during the entire period,'but rather I form an initial .0002" to .0005coat in said electrolyte, and then form the remainder of the coat in an electrolyte at lower temperature e.g., at about 35 F. or less, according to the technique described more fully below. To obtain as smooth a coating as possible, and

if this is the controlling factor, it is desirable to remove the part from the initial electrolyte at the higher temperature as quickly as possible, the thinner the initial coat above about .0002" the better, assuming such thin initial coat is uniform over the entire part. Hence, for ex-. ample, where I desire to form a very smooth oxide coating of say .001" to .00 it is preferable to produce first a .0005" coating in the electrolyte at the higher temperature of about 40 to about 65 F., remove the part from this bath, and proceed to form the remainder of the coating in the electrolyte at lower temperature preferably below 35 F. In this manner a smoother and harder coat is formed than if I form the coating up to the desired .001 to .002 in the initial bath at the higher temperature of about 40 to about 65 F. as described above. It will be further understood that where I desire to form a coating having a thickness greater than about .002", I may form a coating of a thickness any where between .0002 and .002 in the electrolyte at about 40 to about 65 F. and complete the anodizing to the desired thickness at the lower temperature. However, in such a situation, where a coat of improved smoothness is to be formed, it is again preferred to transfer the part to the lower temperature bath as soon as the initial coat, e.g., a .0005" coating, is formed in the initial-bath at the higher temperature of about. 40 to about 65 F.

. The second stage operationinan electrolyte at-lower' temperature is carried out in an. increasing stepwise series of substantially constant voltage steps, said voltage steps. varying from about 2 to about 5 volts between. successive steps, each successive increment of constant voltage being maintained for an interval ranging from about 1 to 3 minutes. Such increments of voltage may be about 2 to 3 volts at first, but such increments may be of greater value as the oxide formation grows.

As a good operating technique the amperage during each voltage step in the second stage operation at lower bath temperature should be permitted to drop materially while maintaining the voltage substantially constant between voltage steps to, for example, about 30% to 50% of the current value initially attained at each such voltage step after the incremental increase in voltage.

At each voltage increment in the second stage operation at the lower temperature the amperage should rise and start to fall after but a small interval of time within about the first 30 seconds after the voltage increment has been applied. If this phenomenon does not occur, the usual consequence is that the amperage will steadily incerase, usually rapidly, and burning results. This is indicative that the voltage increment was too great, unless some'mechanical or electrical failure is the cause of this rise. Thus, the voltage increment should in each case be less than that which permits such excessive current flow. As a practical guide, it is desirable to limit the voltage increment so as to establish a current value not more than about to of the amperage obtained when the previous voltage increment was applied. Preferably, also, the voltage increment should be less than that which gives the burning phenomenon previously described.

At the commencement of each voltage step in the second stage operation, the voltage is increased to reestablish in rough approximation the value of the current observed at the commencement of the previous voltage step. I have observed as practical guide that if the amperage value is dropped to approximately 30% to 50% of the initial value during each voltage'step', that a voltage increment of about 2 or 3 volts is generally sufiicient during the early portions of the second stage operation at lower temperature below about 40 F., to re-establish the aforementioned current value which will give good oxide coating without burning. Subsequent voltage changes are adjusted to re-establish at the initiation of each voltage step increase the amperage found. safe, i.e., in order to obtain deposition without destruction of the oxide coat. During the latter stages of oxide formation to the final coating thickness, voltage increments of say 3 to 5 volts are usually required to approximately re-establish such current value.

Thus, for example, the voltage may be finally increased in the second stage operation to and above 100 volts, and as high as about 130 volts, to wit, to the voltage at which the coating no longer increases. in thickness. During the second stage of the process of this invention, current density is generally maintained at less than 20 amps/sq. it, often dropping below 10 amps/sq. ft. at the end of each of the voltage increments.

Up to about 100 volts the growth of oxide on a particular aluminum alloy is uniform and reproducible. Above 100 volts I have found that the condition of the alloy, i.e., its porosity, grain size and density, aifect the growth materially so that in many cases, similar alloys will react non-uniformly at these high voltages. Thus, while most aluminum alloys will produce a coating say of .006" thickness at 100 volts in my process, the coatings of some will increase to .010" at 130 volts, while other exactly similar alloys difierently treated or handled differently in fabrication will not so improve in growth by an increase in voltage from 100 to 130 volts.

Oxide coatings according to the invention can be prolduced on'various alloys, a few of which are illustrated elow;'

- TABLE 1 Aluminum alloy .3% Cr, 5.6% Zn.

The following examples are given as illustrative but not as limitations of my invention.

a Examplel A 4 'x 4" x /4" test panel of 24S aluminum alloy is connected to the anode of an electrolytic cell compris inga stainless steel tank which forms the cathode. The electrolyte is prepared by adding about 3% by volume of the aqueous extract of Georgia peat produced as described above to a water solution of sulphuric acid formed by the addition to water of about 9% by volume of 66 B'aum sulphuric acid. The temperature of the electrolyte is between about 50 and 55 F.

1 A voltage of about 20 volts is applied, and the ,voltage is gradually increased toabout 26 volts over a perind of aboutl minute. During this period amperage rises from a current density of about 20 amps. per square foot to about 55 amps. per square foot. At the end of this period, voltage is increased one volt for about each 30 to 45 seconds. When the voltage reaches about 34 volts, the coating thickness of .0005" is obtained. While proceeding from 26 volts to 34 volts, amperage at the start of each successive voltage step is lower than at the start of the previous voltage step, and also amperage decreases during each voltage step. During the final voltage step at about 34 volts, amperage ranges from a high of about 20 arnps. per square foot to a low of about amps. per square foot at the end of this voltage step. The total timerequired to produce this .0005" coat is about 5 to 7 minutes.

The part is then removed from the electrolyte and made the anode of a second electrolytic cell having an electrolyte with the same composition as the first bath, but maintained at a temperature of about F. to about 25 F. The voltage is then raised in steps starting at about 35 volts, the successive voltage steps ranging from 2 to 5 volts between steps, each step being maintained substantially constant for a period varying from about 1 to about 3 minutes until a .005" coating is obtained. During this period current density is maintained less than about amps. per square foot, the amperage dropping below this value during each 1 to 3 minutes constant voltage period. The current density at the start of each new increment of voltage ranges from about 10 to about 18 amperes per square foot, the amperage dropping to from about 3 to about 6 amperes per square foot at the end of each voltage increment.

The total time required for producing a .005" coat according to the above procedure is about 45 minutes (5 to 7 minutes in the high temperature bath and about 38 minutes in the low temperature bath). To produce 21.005" thick coating on a similar alloy by the procedure of my co-pending application employing substantially the same electrolyte required about 67 minutes, thus indicating the advantage herein as regards reducing the time of operation for producing a given oxide coat thickness.

Further, the coating produced according to Example '1 above is smoother than a .005 coat formed by the procedure of my co-pending application or by the prior Ex'ampIeZ ji Re sults similar to those noted in Example 1 are obtaiua'ble employing 618 or 758' alloy instead of 124s alloy.- Employing 61S andS alloy panels having an initial "smoothness corresponding to an 8 micro finish, my process, following the procedure of Example 1 up to formation of a coating .001" thick, produces a part having a smoothness not exceeding a 12 micro finish surface for such .001" oxide coating, whereas employing the technique of my co-pending application, wherein the part is anodized throughout at a single low temperature level, the surface exhibits a 24 micro finish at a coating thickness of .001, and the prior art techniques produce a still rougher surface on the order of about a 32 micro finish. The micro finish number is the average root mean square value in micro inches of the height of the bumps or irregularities on a metal surface. See Machinerys Handbook, 12th edition, page 1776, or American Standards Association Bulletins B46.1-1947 and 13462 1951. The lowerthe micro finish value, the smoother is the surface.

Example 3 j The procedure of Example 1 is carried out up through formation of a .0005" oxide coating in the first cell at between about 50 and 55 F. But instead of thereafter transferring the panel to an electrolyte at lower temperature, the panel is kept in the initial electrolyte at about 50-55 'F. and voltage is continued to be increased 1 volt about every 30 to 45 seconds until a coating of .0015" is obtained at about 44 volts. During this period amperage decreases as the thickness of'coat increases, and when a .0015" coat is formed, the current, density is reduced to about 20 amperes per square .foot or less. ;The total time for formation of a .0015 coat in the initial bath at the higher temperature of 50-55 F. is on the order of about 15 minutes.

The panel is then transferred to another electrolyte of the same composition but maintained at about 15 F. to 25 F. The voltage is then raised in steps as in'the second stage of Example 1, beginning at a voltage of between about 45 and 50 volts, until a .005" oxide coat is formed. The .005" coating thus formed has improved smoothness over a .005 coat produced by the prior art, but is not quite as smooth as the .005" coat formed in Example 1, wherein the part has an initial coat of only .0005" when it is transferred to the second ce Instead of using the aqueous extract of peat as additive in the electrolyte employed in the invention process, other additives may be used which permit the technique and procedure of the invention as described above to be applied to the electrolyte for obtaining the results of the invention. Thus, such additives should function to permit'substantially increasing the voltage in the manner described above without substantial increase in current to a value producing burning, while operating at reduced electrolyte temperatures, to obtain oxide coatings which are thick, hard, dense, and smooth, according to the invention. Although I do not wish to be bound by any theoryas to the manner in which these additives function in my process, it appears that the additives act to suppress or inhibit the rate of solution of the oxide coating in the acid electrolyte. The observed facts show that the current efficiency to produce oxide coating is improved by employing these additives in that the oxide coating produced by a given quantity of current is increased when using such additives, and for this reason such additives when used in my process may be termed current efi'iciency improvers. Other suitable additives include, for example, Z-aminoethyl sulphuric acid, taurine, and alkyl taurines, as for example N-methyl taurine and N-cyclo-hexyl taurine, and sulfamic acid. The use of the latter specific compounds as additives in electrolytes for the electrolytic oxidation of aluminum and its alloys is described and claimed in the applications of RobertErnst, Serial Nos. 457,314,457315 and 457,316,

all filed September 20, 1954, now Patents.;2,855,350, 2,855,351 and 2,855,352, respectively. From the foregoing, it is seen that the instant process preferably involving a two stage temperature operation forel'ectrolytic anodizing of aluminum and its alloys provides an improvement over the process of my copending application and the prior art particularly 1n reduction of the time required to form a hard anodic oxide coating of a given thickness on the metal. The process herein described also has the advantage of forming oxide coatings which are generally smoother than those produced by other techniques.

While I have described a particular embodiment of my invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

I claim:

1. A process for coating aluminum and aluminum alloy metal articles with a hard and tough coating of oxide of aluminum, which comprises forming an initial oxide coating on the article by passing an electric current through an electrolytic cell containing an electrolyte maintained at a temperature of between about 40 and about 65 F., with said article forming the anode, said electrolyte comprising a water solution of an electro-anodizing acid, and the procedure for forming said initial oxide coating including increasing the voltage applied across said cell during the formation of said initial coating at a rate which does not cause the current density of said cell to increase, forming an additional oxide'coating by continuing to electrol-ytically-oxidize saidarticle in an electrolyte of the aforementioned composition maintained at a temperature less than about 40 F. but not less than F., the procedure for forming said additional coating including increasing the voltage applied across said cell during the formation of said additional coating at a rate which results in maintaining current densities having values not substantially in excess of that obtained at the end of the initial coating procedure.

2. A process for coating aluminum and aluminum alloy metal articles with a hard and tough coating of oxide of aluminum, which comprises forming an initial oxide coating on the article by passing an electric current through an electrolytic cell containing an electrolyte maintained at a temperature of between about 40 and about 65 F., with said article forming the anode, said electrolyte comprising a water solution of an electro-anodizing acid, and an oxide coating accelerator material, the procedure for forming said initial oxide coating including increasing the voltage applied across said cell during the formation of said initial coating at a rate which does not cause the current density of said cell to increase, forming an additional oxide coating by continuing to electrolytically oxidize said article in an electrolyte of the aforementioned composition maintained at a temperature less than about 40 F. but not less than 0 F., the procedure for forming said additional coating including increasing the voltage applied across said cell during the formation of said additional coating at a rate which results in maintaining current densities having values not substantially in excess of that obtained at the end of the initial coating procedure.

3. A process as set forth in claim 2, wherein the initial oxide coating formed is between about .0002" and .002" thick.

4. A process as set forth in claim 3, wherein said electroanodizing acid is sulfuric acid, and the temperature of the electrolyte during the formation of said additional oxide coating is between about 0 and about 35 F.

5. A process as set forth in claim 3, wherein said coat ing accelerator material is an aqueous extract of peat.

6. A process as set forth in claim 3, wherein said procedure for forming said additional coating is effected by increasing the voltage applied across said cell in increments and maintaining the higher voltage after each such incrementof voltage and oxidizing the metal articleat a decreasing current density while maintaining said higher voltage after each such increment of voltage has been ap-: plied.

7. A process as set forth in claim 3, wherein said coating accelerator material is chosen from the group consisting of an aqueous extract of peat, 2-aminoethyl sul-i furic acid, taurine, N-methyl taurine, N-cyclohexyltaurine, and sulfamic acid.

8. A process as set forth in claim 3, wherein said coat, ing accelerator material is an aqueous extract of peat having been obtained by an extraction of peat with waterat elevated temperatures.

9. A process as set forth in claim3, wherein the voltage; applied across said cell during the formation of saidinitial oxide coating ,is increased ata rate which results in decreasing values of the current density.

10. A process for coating aluminum and aluminum alloy articles with a smooth, hard and tough coating of oxide of aluminum, which comprises in a first stage passingan electriccurrent through an electrolytic cell containingan electrolyte maintained at a temperature of about 40 to about 65 F. with said article forming the anode, said electrolyte comprising a water solution of sulfuric acid and an aqueous extract of peat, said extractbeing obtained by; extracting a mixture of said peat with water at elevated temperature, raising the voltage and amperage gradually, thereafter raising the voltage in increments and decreas-. ing current density until an oxide coating between. about .0005f and .0015" thick is formed, continuing electrolytic:

oxidation in a second stage in an electrolyte of the aforementioned composition maintained ,at a lower temperature than about 40 F. but not less than about 0 F., raisingthe voltage in increments above the last voltage applied in the first stage electrolyte, and maintaining said highervoltage after the addition of each increment of voltage and oxidizing said metal article at a decreasing current value while maintaining said higher voltage after .each' increment of voltage has been applied.

11. The process as defined in claim 10, wherein the temperature of the electrolyte in said second stage is between about 0 and about 35 F.

12. A process for coating aluminum and aluminum alloy articles with a smooth, hard and tough coating of oxide of aluminum, which comprises passing an electric current through a first electrolytic cell containing an electrolyte maintained at a temperature between about 40 F. and about 65 F. with said article forming the anode, said electrolyte comprising a water solution of sulfuric acid and an aqueous extract of peat, said extract being obtained by extracting a mixture of said peat with water at elevated temperature, raising the voltage and amperage gradually to attain a preselected maximum current density, raising the voltage in increments at de creasing overall current density until an oxide coating about .0005" thick is formed, transferring said part to a second eletcrolytic cell containing an electrolyte of the aforementioned composition and maintained at a temperature of between about 0 and 35 F. with said article forming the anode, passing an electric current through said second cell, raising the voltage in increments above the ing current value while maintaining said higher voltage after each increment of voltage has been applied until a desired coating thickness greater than .0005" is formed.

13. A process for coating aluminum and aluminum alloy articles with a smooth, hard and tough coating of oxide of aluminum, which comprises passing an electric current through an electrolytic cell containing an electrolyte maintained at a temperature of about 40 to about 65 F., with said article forming the anode, said electrolyte comprising a water solution of sulfuric acid and an aqueous extract of peat, said extract being obtained by extractmosaics ingaunixtureof. said peat with waterzat elevatedtempera ture,'raising gradually the voltage to about 26 'volts and raising the amperage to a maximum .current density at said 26 volts, thereafter raising the voltage in increments ofabout 1 volt each for a period of from .about 15 .to 45 seconds at decreasing .current density not greater than said maximum value, until an oxide coating of between about 110$":a11d .0015" thick is formed, continuing electrolytic oxidation in an electrolyte of the aforementioned composition maintained at a lower temperature than about 40 F. but not less than about L F., increasing the voltage above the last voltage applied in the electrolyte at higher temperature, by a plurality .of increasing voltage steps, the voltage at each step being maintained substantially constant, and the voltage at one step being increased to dhe next step when the current at said one step decreases substantially.

14. The process as defined :in claim 13, wherein said coating is about .0005" thick, and including continuing electrolytic oxidation in a second stage in an electrolyte of the aforementioned composition maintained at a temperature between about 0 and 35 F., increasing the voltage above the last voltage applied in the electrolyte at about 40-65 F. by a plurality of increasing voltage steps, the voltage at each step being maintained substantially constant, and the voltage at one step being increased to the next step when the current at said one step decreases substantially.

. 15. A process for coating aluminum and aluminum alloy articles with a smooth, hard and tough coating of oxide of aluminum, which comprises passing an electric current through a first electrolytic cell containing an electrolyte maintained at a temperature between about 40 F. and about 65 F. with said article forming the anode, said electrolyte comprising a water solution of sulfuric acid and an aqueous extract of peat, said extract being obtained by extracting a mixture of said peat with water at elevated temperature, raising gradually the volt- 12 agefrom about v20 volts to about 26 volts and raising the amperage to a preselected maximum current density at said 26 volts, and thereafter raising the voltage in in crementsot about 1 volt each for a period of from about 15 045 seconds at an overall decreasing current density not greater than said maximum value, until an oxide coating between about .0005" and about .0015" thick is formed, transferring said article to second electrolytic cell containing an electrolyte of the aforementioned composition and maintained at a temperature of between about 0 and 35 F. with said article forming the anode, passing an electric current through said second cell, increasing the voltage above the last voltage applied in said first electrolytic cell, by a plurality of increasing voltage steps, the voltage at each step being maintained susbtantially constant, and the voltage at one step being increased to the next step when the current at said one step decreases substantially.

16. The process as defined in claim 15, wherein said coating thickness formed in said first electrolytic cell is between about .0005" and about .001" thick.

17. The process as defined in claim 15, wherein the maximum voltage in said first electrolytic cell is about 31 to '34 volts and said maximum current density is from about to amperes per square foot, and a coating of about .0005" thick is-formed in said first cell.

References Cited in-rhe file of this patent UNITED STATES PATENTS 2,174,840 Robinson et a1. Oct. 3, 1939 2,743,221 Sanford Apr. 24, 1956- V FOREIGN PATENTS 716,554 Great Britain Oct. .6, 1954 OTHER REFERENCES Metal Finishing, February 1948, pages through 70, article by Mason et al. 

1. A PROCESS FOR COATING ALMINUM AND ALUMINUM ALLOY METAL ARTICLES WITH A HARD AND TOUGH COATING OF OXIDE OF ALUMINIUM, WHICH COMPRISES FORMING AN INITIAL OXIDE COATING ON THE ARTICLE BY PASSING AN ELECTRIC CURRENT THROUGH AN ELECTROLYTIC CELL CONTAINING AN ELECTROLYTE MAINTAINED AT A TEMPERATURE OF BETWEEN ABOUT 40 AND ABOUT 65*F., WITH SAID ARTICLE FORMING THE ANODE, SAID ELECTROLYTE COMPRISING A WATER SOLUTION OF AN ELECTRO-ANODIZING ACID, AND THE PROCEDURE FOR FORMING SAID INITIAL OXIDE COATING INCLUDING INCREASING THE VOLTAGE APPLIED ACROSS SAID CELL DURING THE FORMATION OF SAID INITIAL COATING AT A RATE WHICH DOES NOT CAUSE THE CURRENT DENSITY OF SAID CELL TO INCREASE, FORMING AN ADDITIONAL OXIDE COATING BY CONTINUING TO ELECTROLYTICALLY OXIDE SAID ARTICLE IN AN ELECTROLYTE OF THE AFOREMENTIONED COMPOSITION MAINTAINED AT A TEMPERATURE LESS THAN ABOUT 40* F. BUT NOT LESS THAN 0*F., THE PROCEDURE FOR FORMING SAID ADDITIONAL COATING INCLUDING INCREASING THE VOLTAGE APPLIED COATING AT A RATE WHICH RESULTS FORMATION OF SAID ADDITIONAL COATING AT A RATE WHICH RESULTS IN MAINTAINING CURRENT DENSITIES HAVING VALUES NOT SUBSTANTIALLY IN EXCESS OF THAT OBTAINED AT THE END OF THE INITIAL COATING PROCEDURE. 